Toxicity of EC and WP Formulations of Insecticides against Susceptible and Phosphine Resistant Strains of Tribolium castaneum

Toxicity of emulsifiable concentrate (EC) and wettable powder (WP) formulations of malathion and two synthetic pyrethroids deltamethrin and bifenthrin were evaluated against susceptible (S) and phosphine resistant (R) strains of Tribolium castaneum by direct spray and film residue methods of bioassay. Against susceptible and phosphine resistant strains, deltamethrin EC was more toxic than malathion EC by 12.2 and 13.6 times by direct spray method and 11.3 and 13.5 times by film residue method, respectively. Lethal concentrations (LC₅₀) of bifenthrin EC and bifenthrin WP, with susceptible strain, obtained by the two methods of bioassay (36.0 and 17.2 by direct spray and 68.1 and 18.3 by film residue) significantly differed from each other. With phosphine resistant strain, LC₅₀ values obtained by two methods (66.8 and 14.1) were significantly different only for bifenthrin EC. Resistance ratio of deltamethrin WP was the lowest among tested insecticides, being 0.84 and 0.82, with direct spray and film residue methods, respectively.